1. Field of the Invention
The present invention relates to the cold finished seamless steel tubes for use of bearing parts such as the retainer ring and the like, more particularly to the same, wherein the dimensional change emerged due to the residual strain at the stage of lathe turning of the outside and inside diameter of the relevant steel tubes is significantly small, and the excellent roundness as well as the superior machinability at the final processing stage of bearing parts are reliably secured.
2. Description of the Related Art
Normally, the seamless steel tubes which require high dimensional accuracy undergo cold working process for precise finish to the targeted outside diameter and wall thickness by either drawing through dies or pilger rolling, subsequent to hot working process by either Mannesmann method, Extrusion method or others being followed on by softening treatment such as spheroidizing anneal and the like.
Accordingly, in-process seamless steel tubes after cold working process are softened by annealing and the like, then subjected to correction process for straightening and securing highly round shape of cross section thereof, and subsequently undergo the inspection for final tube products to be shipped out. The correction process after cold working process is applied to straighten the bend generated in annealing and to rectify the oval-like cross section caused by annealing into round one, by usually utilizing multi-roller straightener or by 2-roll straightener.
The cold finished seamless steel tubes thus shipped out are cut into ring-like work pieces, then are subjected to lathe turning and polishing for the predetermined dimension to be widely used for bearing parts of various equipment, of which the representative examples in use are the retainer ring and the like of the bearing.
When the work pieces for bearing parts are produced by aforementioned cold finishing, the common processing steps from billets to final bearing products are exemplified by the following: hot working process→spheroidizing annealing→cold working process→softening anneal→correction process→mill inspection for shipping→cutting of tubes→lathe turning→quench and temper (heat treatment)→polishing→assembling.
Meanwhile, the steel tubes made by said processing steps generally entail the residual stress due to correction process after cold working and intrinsically come to hold the residual strain in themselves. Accordingly, the thin wall steel tubes that the outside diameter is 70 mm or more and the ratio of the wall thickness to the outside diameter is 10% or less, at the stage of the machining process such as cutting and lathe turning as well as at the stage of quenching as for final heat treatment, are likely to lose the roundness owing to said residual stress, consequently resulting in the dimensional nonconformance.
In case the marked residual stress should exist, the large deformation due to the relevant residual strain is generated, which causes the frequency of polishing operation for dimensional correction to increase significantly, and or arouse the problem of failing in dimensional correction by polishing because of excessive deformation.
In order to cope with these problems, various proposals are made up to this time. For instance, the Japanese Patent Application (Laid-open) Publication No. 10-137850 describes the correction process of seamless steel tubes made by Ugine-Sejournet method for leaving low residual strain by applying an optimal condition such that the electric current of the load onto the upper and lower roll in 2-roll line contact type straightening machine is controlled.
Further, the Japanese Patent Application (Laid-open) Publication No. 2001-329316 discloses that, in order to remove the residual stress built up by the correction process, the annealing is carried out at 520° C. to 630° C. and the marginal correction process by using 2-roll air bend type straightener is applied such that the offset and the crush are controlled at 5 mm or less, and 1.5 mm to 5 mm respectively, which enables to obtain the hot finished seamless steel tubes with less dimensional change at the stage of machining.
Whist, as the high demand for cost reduction in manufacturing bearing parts is recently increased, it is required to reduce the margin of machining in the step of finishing of the work pieces and to secure much more round tubes than ever. And then, the problem arises that the previous proposals above are not adequate to meet these requirements.